The Whirling Frenzy variant can give you an extra attack at lower levels, but it carries a -2 penalty, which actually matters at said lower levels.
For more multiplier stacking insanity, try using a Valorous (UE) lance with Headlong Rush (Also UE) and Spirited Charge from horseback. If my math is correct, that would be another x5 to your charge damage.
AFAIK, however, D&D math (i.e. x2x2=x3, not x4) applieas to damage, though.